Chymotrypsin, labeled with a heavily tritiated diisopropylphosphoryl (DIP) group, was exposed to tritiated hydrogen sulfide (HST), whereupon tritium was preferentially incorporated into residues close to the binding site. The mechanism of this reaction has been studied. An hypothesis was considered, that the reaction involves abstraction of tritium from the DIP group by the sulfhydryl radical. Migration of the resulting carbon radical, followed by the reaction with HST, could then account for the observed labeling effect. It was found that this reaction occurs readily under the conditions of the above labeling reaction, thus adding weight to the hypothesis. Studies will be continued with emphasis on the possible use of this labeling reaction as a means of obtaining more information on binding site residues in general.